1. Field of the Invention
The present invention relates to a diagnostic ultrasound apparatus and, more particularly, relates to a diagnostic ultrasound apparatus in which echo data acquired by radial scanning and represented by a polar coordinate system are converted into echo data represented by a rectangular coordinate system, and then such converted echo data are displayed on a display.
2. Description of the Prior Art
There are known diagnostic ultrasound apparatuses which display tomographic images and the like of a patient based on echo data obtained by transmitting ultrasound waves into the patient and receiving the echo thereof. In the ultrasound apparatus, the transmission and reception of ultrasound waves is carried out with ultrasound probes to perform scanning with ultrasound beams within a scanning plane (i.e., the region from which echo data is obtained). In general, such scanning is performed by mechanical scanning with a transducer or electrical scanning with a transducer array. In recent years, the ultrasound probes of the types which are to be inserted into the blood vessels or body cavities (e.g., the esophagus, digestive tract, urinary tract, vagina, etc.) of patients are widely used.
Furthermore, various ultrasound beam scanning methods have been developed, such as linear scanning, sector scanning, convex scanning, radial scanning and the like. For example, in ultrasound probes of the types which are to be inserted into blood vessels or body cavities, radial scanning is performed to form a circular scanning image. In this case, when such radial scanning is carried out, echo data which have been obtained are identified by a beam number B and a radial direction sampling point number S, respectively, under the polar coordinate system. Thus identified echo data are then temporarily stored in an echo data memory.
However, when an image is actually being displayed on a display, it is necessary to display a circular image identical to that of the scanning plane. For this reason, in order to make the echo data read out from the echo data memory conform with a TV scanning format, a coordinate conversion from the polar coordinate system into the rectangular coordinate system must be carried out at the time such echo data is to be written into a display memory (frame memory).
Hereinafter, the prior art method of processing echo data will be explained with reference to FIG. 1.
As shown in FIG. 1, when radial scanning is being carried out, the echo data obtained from the ultrasound probe are temporarily stored in the echo data memory 10 in each ultrasound beam. In this prior art example, each echo data is identified by a beam number B (.theta. address) and a sampling point number S (R address) thereof, and then thus identified echo data are sequentially stored in the echo data memory 10.
Next, in order to form a display image, the echo data which are read out from the echo data memory 10 undergo a coordinate conversion process from the polar coordinate system into the rectangular coordinate system and thus-converted echo data are written into a display memory 12 (frame memory). As a result, the echo data of each beam is stored in the frame memory 12 in such a way that shows the directions of emissions of such beams. At the time such data are being written into the frame memory 12, an XY address generator 16 generates addresses X, Y sequentially, normally starting from X, Y=0, 0, which are then outputted to the address terminals of the frame memory 12.
In the above arrangement, it is necessary to read out from the echo data memory 10 the echo data which corresponds to the addresses X, Y of the frame memory 12. For this purpose, an address conversion table 14 is provided. Namely, the address conversion table 14 is provided for converting addresses X, Y into addresses R, .theta.. In the address conversion table 14, a table including calculated results of the following formulas is provided: EQU R=(X.sup.2 +Y.sup.2).sup.1/2 EQU .theta.=tan.sup.-1 (Y/X)
By means of the address conversion table 14, the echo data stored in the echo data memory 10 are read out in accordance with the addresses R, .theta. which have been converted from the addresses X, Y. In this regard, it should be noted that R corresponds to the address S of the respective beam and .theta. corresponds to the address B.
Now, after all the echo data have been stored in the frame memory 12, such data are read out therefrom. The read out echo data are D/A converted, and then displayed on a display (not shown in the drawings) in the form of a tomographic image.
Unfortunately, in such prior art diagnostic ultrasound apparatuses, the address conversion table 14 must have all the data between X, Y addresses of the frame memory 12 and the corresponding R, .theta. addresses of the echo data memory 10. Therefore, it is necessary for the address conversion table 14 to have huge storage capacity.
In more details, in the prior art apparatus, the addresses X, Y of the frame memory 12 are inputted into the address conversion table 14, and the corresponding addresses B, S of the echo data memory 10 are outputted therefrom. Accordingly, in the case where the frame memory 12 has an M.times.M matrix size and the size of the echo data memory 10 is such that is includes a total beam number L and a total sampling point number N, the conversion table 14 is comprised of M.times.M words, in which each word being comprised of {log(L.times.N)/log 2} bits.
For example, in the case where M=512, B=1024 and S=256, the address conversion table 14 is comprised of 512.times.512 words, in which each word being comprised of 18 bits. In other words, the address conversion table 14 requires a total storage capacity of 512.times.512.times.18=4.5M bits, and this is a very huge storage capacity.
Such a huge storage capacity of the address conversion table 14 unavoidably leads to an increased size of the apparatus and higher costs for manufacturing thereof. In particular, in the case of diagnostic ultrasound apparatuses for diagnosing blood vessels, they are required to have a reduced size, since they are used in a relatively narrow space such as an operation room. Alternatively, in the case where such address conversion is carried out using an arithmetic circuit instead of such an address conversion table, the address conversion rate becomes slow, thus making it difficult to perform rapid operations with the diagnostic ultrasound apparatus. In this case, it should be understood that since the ultrasound apparatuses for diagnosing blood vessels are required to have high frame rate due to rapid movements of a blood vessel to be diagnosed, they are required to have more rapid data processing speed for processing echo data obtained from reflected ultrasound waves.